The present invention relates to wastewater treatment systems, and more specifically, to those systems requiring a simple and inexpensive means for reducing effluent turbidity and increasing the settleability of suspended solids.
Conventional wastewater treatment systems comprise a primary clarifying means such as a settling tank, a secondary or biodegradative treatment means, such as a trickling filter or rotating biological contactor (RBC), and a secondary clarifying means such as a circular clarifier. Settled solids collected from the primary and secondary clarifiers are treated by dewatering, digestion, drying, etc. prior to ultimate disposal at a landfill or on agricultural lands.
The achievement of rapid and complete settling of suspended solids of both organic and inorganic composition has long been a major objective of treatment plant designers and operators. This dilemma is a result of the often conflicting process requirements of the biodegradative phase and the clarification phase.
For optimum biodegradation, wastewater is normally kept in an agitated, aerated condition to supply a suspending biomass with access to both air and an adequate food supply, the latter usually taking the form of suspended and soluable waste material. However, once the wastewater is transferred to the clarifier, essentially quiescent conditions prevail to enable the waste to settle out as soon and as completely as possible.
Additional means have been devised to enhance the transition between biodegradation and clarification, and to more completely remove suspended inorganic and organic solids from the final plant effluent. The industry term for this process is "the improvement of solids contact."
These additional means normally comprise some form of auxiliary treatment means, such as a holding tank between the biodegradation and clarification stages. The holding tank provides a site to enhance the settleability of wastewater solids without being subject to either the agitated environment of the biodegradation tank, or to disturbing the quiescent environment of the secondary clarifier.
Quite often, in order to remove adequate amounts of suspended matter from the effluent, this additional transition means takes the form of a filter, such as a sand filter similar to that disclosed in U.S. Pat. No. 3,587,861 to Ross, or a rotating microscreen drum of the type described in U.S. Pat. No. 4,038,187 to Saffran having a relatively fine mesh screen.
Unfortunately, these additional measures, be they holding tanks, filters or microscreens, require the construction of expensive tanks and other apparatus and, in the case of sand filters, are expensive to maintain. Thus, cost considerations often place this type of treatment out of the financial reach of many smaller communities.
This problem is compounded when state regulations require that treatment plant effluent not exceed a suspended solids concentration of 5-10 milligrams per litre of effluent. This additional level of treatment quality is more difficult, if not impossible, to achieve by conventional clarification processes. Thus, supplemental solids separation facilities become a costly necessity to treatment plants of all sizes.
Prior efforts to reduce the high cost of auxiliary treatment by combining the process of solids collection with biodegradation are exemplified by U.S. Pat. No. 3,575,849 to Torpey, who discloses the use of a secondary sedimentation tank after an aeration tank. The sedimentation tank is equipped with a plurality of RBC's near the surface, and a solids collection apparatus near the bottom of the tank to collect and transfer settled solids or sludge.
Conventional RBC's are used in secondary biological wastewater treatment processes. RBC's provide surfaces for the growth of a biomass which has the ability to absorb, adsorb, coagulate and oxidize undesirable organic constituents of the wastewater and to change them into unobjectionable forms of matter. The contactors are typically rotated partially submerged in wastewater in a treatment tank so that the surfaces are alternately exposed to the wastewater and to oxygen in the overlying atmosphere. A film of wastewater is carried into the air and trickles down the surfaces of the contactor while absorbing oxygen from the air. Organisms in the biomass remove dissolved oxygen and organic materials from the film of wastewater and unused dissolved oxygen in the wastewater film is mixed with the contents of the mixed liquor in the tank.
Conventional RBC's comprise a plurality of disks mounted on a rigid shaft in parallel orientation to each other and transverse to the flow of the effluent in the tank. The RBC disks are normally positioned to be submerged to one half their diameter in wastewater and the remaining half exposed to the ambient air to promote the growth of organisms on the disks. As the RBC rotates, the disks become covered with a biological slime which absorbs the undesirable constituents of the wastewater.
Torpey continually recirculates a majority of the settled sludge to the aeration tank for reuse in clarifying the wastewater (the excess sludge is conducted to disposal). This is necessary because the settled sludge contains active degradative organisms, which, if permanently removed from the aeration tank, would soon deplete the resident microbial population below the threshold necessary for effective biodegradation.
However, Torpey discloses the installation of RBC's in the clarification tank where they could not help but disrupt the settling process. Despite Torpey's assertions that the placement of the RBC's in the upper regions of the tank would not disrupt settling, experience has shown that disruption does occur. Furthermore, this assertion is indicative of Torpey's emphasis on the maximization of biodegradation, and explains his failure to address the need to enhance the physical settling of solids in the clarifier.
Later work in the RBC field has shown that RBC's are more effective in the secondary treatment tank than in the clarification process, and as such modern RBC's are usually not placed in clarification tanks. See for example U.S. Pat. Nos. 3,596,767 to Antonie and 3,886,074 to Prosser.
It has also become common practice to recirculate sludge from the clarifier back to the secondary treatment RBC tank to replenish the population of degradative biota. See for example U.S. Pat. No. 4,093,539 to Guarino, who, in conventional fashion, introduces recycled sludge at the influent end of the aeration tank. From a biodegradative process standpoint, this is the optimal point of introduction. However, introduction at this point does not enhance solids contact, or the settling of suspended fine particles.
It is therefore a major objective of the present invention to provide a wastewater treatment apparatus which enhances solids settling, increases solids contact and lowers turbidity in the final effluent.
It is a further objective of the present invention to provide a wastewater treatment apparatus capable of increasing solids contact without requiring the construction of extensive additional facilities.
It is a still further objective of the present invention to provide a method of wastewater treatment whereby the step of tertiary filtration may be eliminated.